1. Field of the Invention
The present invention relates to a radiation imaging apparatus and a radiation imaging system.
2. Description of the Related Art
In recent years, radiation imaging apparatuses including a flat panel detector (hereinafter abbreviated as “FPD”) formed of a semiconductor material have started to be put to practical use as radiation imaging apparatuses that used for medical diagnostic imaging or a nondestructive inspection using an X-ray. In an FPD, a plurality of pixels having conversion elements constructed using semiconductor material such as a-Si that is capable of converting radiation into electric charges and switching elements that transfer electric signals that correspond to the electric charges are arranged two-dimensionally. Such radiation imaging apparatuses including an FPD are used, for example, in medical diagnostic imaging as digital imaging apparatuses for still image radiographing like general radiographing or moving image radiographing such as fluoroscopic radiographing.
When performing radiographing, a radiation imaging apparatus performs radiographing in synchrony with the operations of a radiation generating apparatus. As synchronization methods, for example, a method is available in which the radiation generating apparatus and the radiation imaging apparatus are synchronized by electrically connecting the two apparatuses to each other, or a method is available in which the radiation imaging apparatus is synchronized with the radiation generating apparatus by detecting radiation that is radiated from the radiation generating apparatus. In the former case, because service personnel connect the radiation generating apparatus and the radiation imaging apparatus with a cable, the connection work involves time and labor, and furthermore the radiation generating apparatus and the radiation imaging apparatus must be fixed and used as a single pair of apparatuses. In the latter case, a method is known in which a radiation detector is provided inside and outside the radiation imaging apparatus, or in which the radiation imaging apparatus itself performs detection of radiation. In this case, there is the advantage that time and labor for connection work are not required, and the radiation imaging apparatus is portable and can be used in combination with various radiation generating apparatuses.
Normally, in an FPD, pixels that include a photoelectric conversion element and a switching element are two-dimensionally arrayed, and reading of signals from the photoelectric conversion elements and resetting of the photoelectric conversion elements is performed in row units. Before radiation is irradiated, the switching elements are subjected to on/off control in row units, and a dark current component flowing to the photoelectric conversion elements is reset. Hereunder, this operation is referred to as an “initializing operation”. If a radiation irradiation signal is received or radiation is detected during an initializing operation, it is necessary to immediately end the operation to reset the photoelectric conversion elements and transition to an accumulation operation. If a transition is not made to the accumulation operation even though a radiation irradiation signal has been received, a time lag will arise between the time that the user pushed the exposure button and the actual photographed image, and an unintended image in which a difference in level or the like arises will be obtained. Further, if an initializing operation is continued even though radiation was detected, since radiation signals that are generated at the photoelectric conversion elements will be reset, unnecessary radiation will have been irradiated at the subject and the amount of radiation exposure may increase.
Japanese Patent Application Laid-Open No. H11-151233 discloses technology that includes a radiation detection unit and that controls operations of a radiation imaging apparatus based on a signal from the radiation detection unit. For example, when the start of irradiation of radiation is determined based on a signal from the radiation detection unit, the operating state of a radiation imaging unit immediately shifts from a radiographing preparation state to an accumulation state, and when the end of irradiation of radiation is determined thereafter, the operating state of the radiation imaging unit immediately shifts from the accumulation state to an image data output state. Further, Japanese Patent Application Laid-Open No. 2010-268171 discloses technology that detects a current that flows through a bias wire that supplies a bias voltage to a radiation detecting element to control the operations of a radiation imaging apparatus. However, with the technology disclosed in Japanese Patent Application Laid-Open No. H11-151233, in some cases it is not possible for the radiation detection unit to adapt to various radiographing kinds (the radiographing site and the build of a subject, a moving image or a still image radiographing mode and the like), and sometimes the radiation detection unit can not accurately detect both the start and end of radiation exposure. Therefore, a radiation detection unit is demanded that is capable of accurately detecting both the start and end of irradiation of radiation with respect to various irradiation conditions.